Inkjet printing mechanisms use cartridges, often called "pens," which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in linear arrays usually located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction, with the length of the nozzle arrays defining a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The width of this band is known as the "swath width" of the pen, the maximum pattern of ink which can be laid down in a single pass. The media is moved through the printzone, typically one swath width at a time, although some print schemes move the media incrementally by for instance, halves or quarters of a swath width for each printhead pass to obtain a shingled drop placement which enhances the appearance of the final image.
The picking and movement of print media through the printzone of an inkjet printing mechanism is the subject addressed herein. The print media, may be any type of substantially flat material, such as plain paper, specialty paper, card-stock, fabric, transparencies, foils, mylar, etc., but the most common type of medium is paper. For convenience, we will discuss printing on paper as a representative example of these various types of print media. The media may be supplied to the printing mechanism in a variety of different configurations. For instance, in desktop inkjet printers, paper is typically supplied in a stack of cut-sheets, such as letter size, legal size, or A-4 size paper, which are placed in an input tray. Typically, sheets are sequentially pulled from the top of the stack and printed on, after which they are deposited in an output tray. Other types of inkjet printing mechanisms feed the paper from a continuous roll, such as an inkjet plotter. Upon completion of plotting an image or drawing on a portion of the continuous roll, the plotter has a severing mechanism to cut the newly printed sheet from the remainder of the roll.
It would be desirable to have an inkjet printing mechanism which can print on both Z-fold media and conventional cut-sheets of media. A Z-fold or accordion folded stack of media has each sequential sheet joined to the adjacent sheet along a fold, with the sheets being bent back onto one another into a Z-shape when viewed from the side. Along each side, conventional Z-fold paper has border extensions with a series of evenly-spaced holes therethrough which are engaged by sprockets of a tractor-feed mechanism on the printer to advance the media through the printzone. Typically Z-fold paper came supplied in a letter sized stack, with perforations along the folds at the top and bottom of each sheet to assist in separating the sheets upon completion of the print job. The border extensions with the tractor feed holes are also joined to the side edges of the media at perforations, which enables separation of the borders from the sheet upon completion of the print job. Unfortunately, the tractor-feed mechanisms were very expensive to build, and often noisy in operation. Furthermore, most of these tractor-fed printers were bulky, increasing the overall size or "footprint" of the printer, so excessive desk top space in the work environment was occupied by these earlier printers.
Yet it would be desirable to use Z-fold paper in a conventional cut-sheet inkjet printing mechanism without a costly tractor-feed. Z-fold media is particularly useful for printing banners, extended graphs, continuous scrolls or outlines of text, and a variety of other images, such as artwork and the like. The versatility of an inkjet printing mechanism would be greatly enhanced if it could feed not only cut-sheets of paper but also Z-fold media. Unfortunately, conventional inkjet printing mechanisms are unable to feed a Z-fold stack of paper from a cut-sheet input tray. By tearing the border extensions off of a Z-fold paper stack, the Z-fold paper will fit in the input tray, but conventional inkjet printing mechanisms are unable to pick the Z-fold media from the tray. Because the Z-fold sheets are physically attached to one another, often the conventional printer tries to pick the entire stack all at once, leading to a significant paper jam. This problem is often encountered in cut-sheet media feeding, and is known in the art as a "multiple pick," where several sheets are picked from the input tray all at once.
For cut-sheet media, this multiple pick problem is often remedied by using a friction separator pad at the edge of the input tray, where media begins to enter the feed zone. The media drive rollers feed the sheet through the feed zone. If the second sheet from the top of the stack moves with the first sheet, the second sheet is driven over a friction separator pad. The coefficient of friction of the friction separator pad to the media is higher than the coefficient of friction between the two media sheets. Thus, the second sheet stops on the separator pad and does not continue to be fed through the mechanism. This prevents a multiple pick. Unfortunately, this conventional manner of preventing multiple picks with cut-sheet media does not work with a Z-fold stack of media because the sheets are all attached, and the first sheet pulls in the second sheet, the third sheet, etc.
For cut-sheet media, sheets left on the separator pad are pushed off the separator pad by a kicker. As the first sheet moves through the feed zone, the trailing edge of the first sheet eventually passes across the feed zone entrance. This trailing edge releases or activates the kicker which pushes the second sheet off of the separator pad and back into the input tray. Without a kicker, the number of multiple picks would increase. For instance, if this partially fed second sheet was not kicked back and the operator added more media on top of the existing media in the input tray, then a multiple pick usually occurs near this remaining partially fed sheet and the new media which has been loaded on top of it. Thus, kickers play an important role in preventing multiple picks when using cut-sheet media. Unfortunately, this conventional kicker method of pushing media off the friction separator pad is totally ineffective to prevent Z-fold media multiple picks. Since the kicker is not mechanically activated until the trailing edge of the last sheet passes through the feed zone entrance, any multiple picks of the Z-fold stack have already occurred when the kicker is finally activated. Thus, the kicker has no function in Z-fold media picking.
Other solutions were also tried to feed Z-fold media. An earlier system tested by the inventors used a hinged guide wall that was elevated by a user when feeding Z-fold paper. Unfortunately, this system was extremely cumbersome. This system required removal of the output tray, and an elaborate threading scheme to insert the leading edge of the Z-fold stack into the media pick area. This loading technique was complex and not very "user friendly." It required a good degree of manual dexterity to thread the media, and it was not intuitive or easy to remember. Most users want to see their image printed, and they do not want to be bothered by elaborate and time-consuming media loading schemes.
Thus, a need exists for a versatile, compact and economical inkjet system mechanism, capable of feeding both cut-sheets of media and Z-fold media, which is quiet and easy to use.